I was looking at the GK MB800 Fusion which is 12.25" wide (wider than TH500) 10.75" deep (more than 2" deeper than TH 500) and 1.75 (which, in fact, is thinner than the TH500). Now, I haven't seen them side by side in a store, they LOOK the same in terms of face layout, but I guess the regular MB800 is actually smaller? Could be, I don't know. I just know the Fusion MB800 is bigger than the TH500, perhaps not the reg'lar MB800. If your specs for the reg'lar are correct, then you are correct that they are the same size (just not the Fusion version).
I like the super portability of the Aguilar TH's, which is why I have the TH 350, which is just tiny, considerably smaller than the 500. I could slip that sucker in the pouch of my bass gig bag. There are not too many cases when the 500 would carry a gig that the 350 would not.
Nevertheless, I wouldn't mind having a GK Fusion MB800 Fusion. Super nice amp and I like the contour knob. It still would not fulfill the role the TH 350 does. As some have said, I think you'd see a lot bigger difference in tone and volume by having 2 112 cabs instead of one 112 cab, than in the difference between a TH 500 (or 350) and the GK head. You need speaker surface area. Final thought on big volume from small sources, the TH 350 has one speaker out instead of two; you daisy chain the 2 SL-112's which lowers impedance to 4ohms. Thus, by doubling up on 12" cabs I have not only doubled surface area using identical cabs which gives deeper bass response, I have added power and efficiency by the impedance change. It really works great together, TH 350 and 2 SL-112's. Did I mention Sl-12's are 25 pounds, lightest on the market? Sweet power and easy load.